The present invention relates generally to transfer of data using Digital Subscriber Loop (DSL) technology, and specifically to a method for reducing transmitter power used for the transfer.
Remote access and retrieval of data is becoming increasingly popular in data communication. The proliferation of the Internet has provided a vast network of information that is available to the general public. As the Internet grows and technology advances, this information is becoming increasingly voluminous and the details are becoming increasingly intricate. What used to comprise mainly text information has grown to include still and moving images as well as sound. The increase in the volume of information to be transferred has presented a need for a high-speed Internet connection, since traditional telephone modems communicate at speeds too slow for efficient communication.
One proposal for high-speed communication is the introduction of Digital Subscriber Line (DSL) technology. The various DSL technologies include asymmetric DSL (ADSL), high-speed DSL (HDSL), symmetric DSL (SDSL), Symmetric High-Bit-Rate DSL (SHDSL) and Integrated Services Digital Network (ISDN) Basic Rate Interlace (BRI) DSL systems. One of the most attractive features of DSL is that it is implemented using an infrastructure that already exists. DSL shares copper twisted pair lines typically used for telephone communication. However, only a small portion of the available bandwidth of the twisted pair line (0 to 4 kHz) is used for Plain Old Telephone Service (POTS). DSL takes advantage of the available frequency spectrum from 4 kHz to approximately 1.1 MHz for transmitting data.
ADSL is currently the most practical form of DSL technology, and therefore the most widely implemented. ADSL is asymmetric in that its downstream (DS or D/S—to a subscriber) capacity is larger than its upstream (US or U/S—from the subscriber) capacity. An ADSL transceiver unit at a central office or remote loop carrier (ATU-C) is used for sending downstream information and receiving upstream information. An ADSL transceiver unit at a remote location or user end (ATU-R) is used for receiving downstream information and sending upstream information. Typically, a Discrete Multi-tone (DMT) scheme is used. The spectrum from 4 kHz to 1.1 MHz is divided into 256 sub-channels, or tones, each having a bandwidth of 4.3125 kHz. Each sub-channel uses Quadrature Amplitude Modulation (QAM) to carry 2 to 15 bits/QAM symbol.
A predefined power level is used by the ATU-C for transmitting the downstream signal. At the ATU-R, the achievable downstream rate is a function of the received signal level and of the receiver input-referenced noise levels. Where the loop attenuation is modest and the target downstream rate is lower than the achievable rate, the predefined ATU-C transmission level can be much higher than necessary. Therefore, an unnecessary amount of power is consumed by the ATU-C and additional crosstalk noise is induced in adjacent DSL lines. Reducing the ATU-C transmit level would both save power at the ATU-C and reduce crosstalk noise, improving the quality of signal on adjacent lines. From a crosstalk perspective, it is beneficial if the ATU-C transmit power can be reduced during both initialization and steady state (also referred to as “Showtime”) operation. If the transmit power and resulting crosstalk is only reduced on entry to Showtime, the higher crosstalk levels during initialization may cause excessive errors and even force re-initialization on adjacent DSL lines.
The benefit of such a power cutback can be significant in Digital Loop Carrier (DLC) applications, for example, where subscriber loops are typically shorter than seen at central office (CO)-resident line interfaces and where power and thermal budgets are tight. Assuming a DMT transmit signal with peak-to-average-ratio (PAR) of 14.5 dB, downstream power cutback can yield significant savings on line driver power alone.
American National Standards Institute (ANSI) and International Telecommunications Union (ITU) compliant ATU-Cs reduce their transmitter power on very short loops to avoid overloading the ATU-R. This reduction in downstream transmission power is often referred to as a “politeness cutback”. The politeness cutback is determined in accordance with a loop loss estimate from an upstream signal path and applies on loops shorter than approximately 2-3 kft 26AWG-equivalent. Other downstream transmitter power cutbacks are also specified in the ITU splitterless ADSL standard (G.992.2, which is also known as G.lite) for reducing the downstream signal level to the ATU-R. This cutback is designed to address ADSL signal levels that could cause distortion in the presence of an off-hook telephone set.
However, even with the implementation of the politeness cutback and off-hook cutback, the ATU-C often transmits at a greater power than necessary and there is no agreed-upon mechanism to implement a general downstream power cutback. It is an object of the present invention to obviate or mitigate at least some of the above-mentioned disadvantages.